Genetic polymorphisms that affect xenobiotic metabolism or cellular response to DNA damage can modulate individual sensitivity to genotoxins. Information on the effects of such polymorphisms on the level of chromosome damage may facilitate the identification of risk groups and increase the sensitivity of cytogenetic endpoints as biomarkers of genotoxic exposure and effect. Glutathione S-transferase M1 (GSTM1) is an important detoxification enzyme which, due to a homozygous gene deletion (null genotype), is lacking from about 50% of Caucasians. A higher level of DNA adducts and chromosome damage has been detected in lymphocytes of tobacco smokers and bus drivers who lack the GSTM1 gene. Other polymorphic glutathione S-transferases include GSTM3, GSTP1, and GSTT1. The GSTT1 null genotype (10-20% of Caucasians) has been associated with an increased "baseline" level of sister chromatid exchanges (SCEs) in lymphocytes. N-acetyltransferase 2 (NAT2), metabolizing xenobiotics with primary aromatic amine and hydrazine structures, is another important polymorphic phase II enzyme. Subjects having the NAT2 slow acetylator genotype appear to show an increased baseline frequency of lymphocyte CAs in the absence of identified environmental exposure. Besides human biomonitoring studies, genetic polymorphisms may be important in explaining individual variation in genotoxic response observed in genetic toxicology tests with human cells. Several studies have suggested that blood cultures from GSTT1 null and GSTM1 null individuals have increased in vitro sensitivity to various genotoxins. The best-known example is probably the diepoxybutane sensitivity of GSTT1 null donors. Recently discovered polymorphisms affecting DNA repair may be expected to be of special importance in modulating genotoxic effects; the first available studies have suggested that the exon 10 Arg399Gln polymorphism of XRCC1 gene (X-ray repair cross-complementing group 1) could affect individual genotoxic response. In conclusion, the genetic polymorphism of GSTM1 influences the frequency of chromosome damage in exposed humans, while that of GSTT1 and NAT2 affect the "baseline" level of such damage. Both GSTM1 and GSTT1 genotypes may shape the in vitro genotoxic response of human lymphocytes. The significance of DNA repair polymorphisms is presently unclear.